Corrugated board or container board is a versatile and relatively inexpensive packaging medium, and is widely used for transportation and storage of goods. Corrugated paper board container is commonly prepared by first forming a corrugated element or medium by passing a cellulosic sheet between corrugating rolls forming a substantially sinusoidal or serpentine cross-section in the sheet. An adhesive is commonly applied to the tips of the thus formed sinusoidal portion called flutes and a noncorrugated or planar cellulosic liner is applied against the adhesive coated flutes of the corrugated elements as the corrugated sheet passes between a corrugating roll and a pressure roll. The resulting paper product having the corrugating medium on one side and the planar liner on another side is called a single-faced web. The single-faced element may be used, as is, in certain applications as a liner or buffer material within a container. More commonly, adhesive is applied to the flute tips of the single-faced web and a second liner sheet is subsequently applied to the fluted medium in a “double facer” operation. The second liner sheet is exposed to conditions of heat and pressure during its contact with the adhesive. In practice, the sheet of corrugated cardboard most frequently encountered has two plane sides placed on each side of the corrugated medium. Depending on the specific strength desired, a sheet of corrugated cardboard may also be provided with a more complex structure, such as two corrugated medium and three plane surfaces, two outer ones and one inner one separating the two corrugated medium. This is referred to as double wall board.
Starch-based adhesives are most commonly used in the corrugating process due to their desirable adhesive properties, low cost and ease of preparation. The most fundamental starch corrugating adhesive, commonly referred to as a “Stein-Hall” formulation, is an alkaline adhesive which is comprised of raw, ungelatinized starch suspended in an aqueous dispersion of cooked starch. The adhesive is produced by gelatinizing starch in water with sodium hydroxide (caustic soda) to yield a primary mix of gelatinized or cooked carrier, which is then slowly added to a secondary mix of raw (ungelatinized) starch, borax and water to produce the fully formulated adhesive. In conventional corrugating processes, the adhesive is applied to the tips of the fluted paper medium or single-faced board, whereupon the application of heat and pressure causes the raw starch to gelatinize, resulting in an instantaneous increase in viscosity and formation of the adhesive bond. In other words, the adhesive is applied while relatively cool and then, subsequently, requires the application of high temperatures at the points of bonding to effect in situ gelatinization of the raw starch granules.
Ordinary corrugated fiberboard containers manufactured in a Stein-Hall adhesive are not resistant to moisture. While water resistant additives, such as ketone-aldehyde have been added to Stein-Hall adhesives to increase moisture- and water-resistance, such addition is insufficient when the boards are exposed to conditions of high humidity and prolonged moisture levels. Water-repellency and water resistance have, therefore, been imparted to corrugated fiberboard sheets to be used for making containers for packaging, storing and/or transporting moisture-containing foods such as fruits, vegetables, fresh seafood, meats and frozen foods, as well as contents which must be maintained at a low temperature by impregnating them with a wax or wax-based material, for example, paraffin wax, polyethylene wax, carnauba wax and lanolin. The most widely used wax in corrugated board is paraffin wax. The most common application methods for applying wax for corrugated board are saturating or cascading and curtain coating.
Unfortunately, wax treated containers cannot be repulped, and such containers must be incinerated or disposed of in a landfill after use. Disposal is also economically disadvantageous, since used corrugated containers that are suitable for repulping have a significant resale value.
There continues to be a need in the art to eliminate or at least minimize the amount of wax required to prepare water-resistant corrugated fiberboard having a high water resistance and capable of being recycled after the use. The invention fulfills this need.